HTTP Authentication: Basic and Digest Access Authentication

This specification is a companion to the HTTP/1.1 specification [2]. It uses the augmented BNF section 2.1 of that document, and relies on both the non-terminals defined in that document and other aspects of the HTTP/1.1 specification.¶

HTTP provides a simple challenge-response authentication mechanism that MAY be used by a server to challenge a client request and by a client to provide authentication information. It uses an extensible, case-insensitive token to identify the authentication scheme, followed by a comma-separated list of attribute-value pairs which carry the parameters necessary for achieving authentication via that scheme.¶

The 401 (Unauthorized) response message is used by an origin server to challenge the authorization of a user agent. This response MUST include a WWW-Authenticate header field containing at least one challenge applicable to the requested resource. The 407 (Proxy Authentication Required) response message is used by a proxy to challenge the authorization of a client and MUST include a Proxy-Authenticate header field containing at least one challenge applicable to the proxy for the requested resource.¶

Note: User agents will need to take special care in parsing the WWW-Authenticate or Proxy-Authenticate header field value if it contains more than one challenge, or if more than one WWW-Authenticate header field is provided, since the contents of a challenge may itself contain a comma-separated list of authentication parameters.¶

The realm directive (case-insensitive) is required for all authentication schemes that issue a challenge. The realm value (case-sensitive), in combination with the canonical root URL (the absoluteURI for the server whose abs_path is empty; see section 5.1.2 of [2]) of the server being accessed, defines the protection space. These realms allow the protected resources on a server to be partitioned into a set of protection spaces, each with its own authentication scheme and/or authorization database. The realm value is a string, generally assigned by the origin server, which may have additional semantics specific to the authentication scheme. Note that there may be multiple challenges with the same auth-scheme but different realms.¶

A user agent that wishes to authenticate itself with an origin server--usually, but not necessarily, after receiving a 401 (Unauthorized)--MAY do so by including an Authorization header field with the request. A client that wishes to authenticate itself with a proxy--usually, but not necessarily, after receiving a 407 (Proxy Authentication Required)--MAY do so by including a Proxy-Authorization header field with the request. Both the Authorization field value and the Proxy-Authorization field value consist of credentials containing the authentication information of the client for the realm of the resource being requested. The user agent MUST choose to use one of the challenges with the strongest auth-scheme it understands and request credentials from the user based upon that challenge.¶

• Note that many browsers will only recognize Basic and will require that it be the first auth-scheme presented. Servers should only include Basic if it is minimally acceptable.

The protection space determines the domain over which credentials can be automatically applied. If a prior request has been authorized, the same credentials MAY be reused for all other requests within that protection space for a period of time determined by the authentication scheme, parameters, and/or user preference. Unless otherwise defined by the authentication scheme, a single protection space cannot extend outside the scope of its server.¶

If the origin server does not wish to accept the credentials sent with a request, it SHOULD return a 401 (Unauthorized) response. The response MUST include a WWW-Authenticate header field containing at least one (possibly new) challenge applicable to the requested resource. If a proxy does not accept the credentials sent with a request, it SHOULD return a 407 (Proxy Authentication Required). The response MUST include a Proxy-Authenticate header field containing a (possibly new) challenge applicable to the proxy for the requested resource.¶

The HTTP protocol does not restrict applications to this simple challenge-response mechanism for access authentication. Additional mechanisms MAY be used, such as encryption at the transport level or via message encapsulation, and with additional header fields specifying authentication information. However, these additional mechanisms are not defined by this specification.¶

Proxies MUST be completely transparent regarding user agent authentication by origin servers. That is, they must forward the WWW-Authenticate and Authorization headers untouched, and follow the rules found in section 14.8 of [2]. Both the Proxy-Authenticate and the Proxy-Authorization header fields are hop-by-hop headers (see section 13.5.1 of [2]).¶

The Digest Access Authentication scheme is conceptually similar to the Basic scheme. The formats of the modified WWW-Authenticate header line and the Authorization header line are specified below. In addition, a new header, Authentication-Info, is specified.¶

Upon receiving the Authorization header, the server may check its validity by looking up the password that corresponds to the submitted username. Then, the server must perform the same digest operation (e.g., MD5) performed by the client, and compare the result to the given request-digest value.¶

Note that the HTTP server does not actually need to know the user's cleartext password. As long as H(A1) is available to the server, the validity of an Authorization header may be verified.¶

The client response to a WWW-Authenticate challenge for a protection space starts an authentication session with that protection space. The authentication session lasts until the client receives another WWW-Authenticate challenge from any server in the protection space. A client should remember the username, password, nonce, nonce count and opaque values associated with an authentication session to use to construct the Authorization header in future requests within that protection space. The Authorization header may be included preemptively; doing so improves server efficiency and avoids extra round trips for authentication challenges. The server may choose to accept the old Authorization header information, even though the nonce value included might not be fresh. Alternatively, the server may return a 401 response with a new nonce value, causing the client to retry the request; by specifying stale=TRUE with this response, the server tells the client to retry with the new nonce, but without prompting for a new username and password.¶

Because the client is required to return the value of the opaque directive given to it by the server for the duration of a session, the opaque data may be used to transport authentication session state information. (Note that any such use can also be accomplished more easily and safely by including the state in the nonce.) For example, a server could be responsible for authenticating content that actually sits on another server. It would achieve this by having the first 401 response include a domain directive whose value includes a URI on the second server, and an opaque directive whose value contains the state information. The client will retry the request, at which time the server might respond with a 301/302 redirection, pointing to the URI on the second server. The client will follow the redirection, and pass an Authorization header , including the <opaque> data.¶

As with the basic scheme, proxies must be completely transparent in the Digest access authentication scheme. That is, they must forward the WWW-Authenticate, Authentication-Info and Authorization headers untouched. If a proxy wants to authenticate a client before a request is forwarded to the server, it can be done using the Proxy-Authenticate and Proxy-Authorization headers described in Section 3.6 below.¶

It is useful for a server to be able to know which security schemes a client is capable of handling.¶

It is possible that a server may want to require Digest as its authentication method, even if the server does not know that the client supports it. A client is encouraged to fail gracefully if the server specifies only authentication schemes it cannot handle.¶

The following example assumes that an access-protected document is being requested from the server via a GET request. The URI of the document is "http://www.nowhere.org/dir/index.html". Both client and server know that the username for this document is "Mufasa", and the password is "Circle Of Life" (with one space between each of the three words).¶

The digest authentication scheme may also be used for authenticating users to proxies, proxies to proxies, or proxies to origin servers by use of the Proxy-Authenticate and Proxy-Authorization headers. These headers are instances of the Proxy-Authenticate and Proxy-Authorization headers specified in sections 10.33 and 10.34 of the HTTP/1.1 specification [2] and their behavior is subject to restrictions described there. The transactions for proxy authentication are very similar to those already described. Upon receiving a request which requires authentication, the proxy/server must issue the "407 Proxy Authentication Required" response with a "Proxy-Authenticate" header. The digest-challenge used in the Proxy-Authenticate header is the same as that for the WWW-Authenticate header as defined above in Section 3.2.1.¶

The client/proxy must then re-issue the request with a Proxy-Authorization header, with directives as specified for the Authorization header in Section 3.2.2 above.¶

On subsequent responses, the server sends Proxy-Authentication-Info with directives the same as those for the Authentication-Info header field.¶

Note that in principle a client could be asked to authenticate itself to both a proxy and an end-server, but never in the same response.¶

The Basic authentication scheme is not a secure method of user authentication, nor does it in any way protect the entity, which is transmitted in cleartext across the physical network used as the carrier. HTTP does not prevent additional authentication schemes and encryption mechanisms from being employed to increase security or the addition of enhancements (such as schemes to use one-time passwords) to Basic authentication.¶

The most serious flaw in Basic authentication is that it results in the essentially cleartext transmission of the user's password over the physical network. It is this problem which Digest Authentication attempts to address.¶

Because Basic authentication involves the cleartext transmission of passwords it SHOULD NOT be used (without enhancements) to protect sensitive or valuable information.¶

A common use of Basic authentication is for identification purposes -- requiring the user to provide a user name and password as a means of identification, for example, for purposes of gathering accurate usage statistics on a server. When used in this way it is tempting to think that there is no danger in its use if illicit access to the protected documents is not a major concern. This is only correct if the server issues both user name and password to the users and in particular does not allow the user to choose his or her own password. The danger arises because naive users frequently reuse a single password to avoid the task of maintaining multiple passwords.¶

If a server permits users to select their own passwords, then the threat is not only unauthorized access to documents on the server but also unauthorized access to any other resources on other systems that the user protects with the same password. Furthermore, in the server's password database, many of the passwords may also be users' passwords for other sites. The owner or administrator of such a system could therefore expose all users of the system to the risk of unauthorized access to all those sites if this information is not maintained in a secure fashion.¶

Basic Authentication is also vulnerable to spoofing by counterfeit servers. If a user can be led to believe that he is connecting to a host containing information protected by Basic authentication when, in fact, he is connecting to a hostile server or gateway, then the attacker can request a password, store it for later use, and feign an error. This type of attack is not possible with Digest Authentication. Server implementers SHOULD guard against the possibility of this sort of counterfeiting by gateways or CGI scripts. In particular it is very dangerous for a server to simply turn over a connection to a gateway. That gateway can then use the persistent connection mechanism to engage in multiple transactions with the client while impersonating the original server in a way that is not detectable by the client.¶

Digest Authentication does not provide a strong authentication mechanism, when compared to public key based mechanisms, for example. However, it is significantly stronger than (e.g.) CRAM-MD5, which has been proposed for use with LDAP [10], POP and IMAP (see RFC 2195 [9]). It is intended to replace the much weaker and even more dangerous Basic mechanism.¶

Digest Authentication offers no confidentiality protection beyond protecting the actual password. All of the rest of the request and response are available to an eavesdropper.¶

Digest Authentication offers only limited integrity protection for the messages in either direction. If qop=auth-int mechanism is used, those parts of the message used in the calculation of the WWW-Authenticate and Authorization header field response directive values (see Section 3.2 above) are protected. Most header fields and their values could be modified as a part of a man-in-the-middle attack.¶

Many needs for secure HTTP transactions cannot be met by Digest Authentication. For those needs TLS or SHTTP are more appropriate protocols. In particular Digest authentication cannot be used for any transaction requiring confidentiality protection. Nevertheless many functions remain for which Digest authentication is both useful and appropriate. Any service in present use that uses Basic should be switched to Digest as soon as practical.¶

The Digest scheme uses a server-specified nonce to seed the generation of the request-digest value (as specified in Section 3.2.2.1 above). As shown in the example nonce in Section 3.2.1, the server is free to construct the nonce such that it may only be used from a particular client, for a particular resource, for a limited period of time or number of uses, or any other restrictions. Doing so strengthens the protection provided against, for example, replay attacks (see 4.5). However, it should be noted that the method chosen for generating and checking the nonce also has performance and resource implications. For example, a server may choose to allow each nonce value to be used only once by maintaining a record of whether or not each recently issued nonce has been returned and sending a next-nonce directive in the Authentication-Info header field of every response. This protects against even an immediate replay attack, but has a high cost checking nonce values, and perhaps more important will cause authentication failures for any pipelined requests (presumably returning a stale nonce indication). Similarly, incorporating a request-specific element such as the Etag value for a resource limits the use of the nonce to that version of the resource and also defeats pipelining. Thus it may be useful to do so for methods with side effects but have unacceptable performance for those that do not.¶

Both Digest and Basic Authentication are very much on the weak end of the security strength spectrum. But a comparison between the two points out the utility, even necessity, of replacing Basic by Digest.¶

The greatest threat to the type of transactions for which these protocols are used is network snooping. This kind of transaction might involve, for example, online access to a database whose use is restricted to paying subscribers. With Basic authentication an eavesdropper can obtain the password of the user. This not only permits him to access anything in the database, but, often worse, will permit access to anything else the user protects with the same password.¶

By contrast, with Digest Authentication the eavesdropper only gets access to the transaction in question and not to the user's password. The information gained by the eavesdropper would permit a replay attack, but only with a request for the same document, and even that may be limited by the server's choice of nonce.¶

A replay attack against Digest authentication would usually be pointless for a simple GET request since an eavesdropper would already have seen the only document he could obtain with a replay. This is because the URI of the requested document is digested in the client request and the server will only deliver that document. By contrast under Basic Authentication once the eavesdropper has the user's password, any document protected by that password is open to him.¶

Thus, for some purposes, it is necessary to protect against replay attacks. A good Digest implementation can do this in various ways. The server created "nonce" value is implementation dependent, but if it contains a digest of the client IP, a time-stamp, the resource ETag, and a private server key (as recommended above) then a replay attack is not simple. An attacker must convince the server that the request is coming from a false IP address and must cause the server to deliver the document to an IP address different from the address to which it believes it is sending the document. An attack can only succeed in the period before the time-stamp expires. Digesting the client IP and time-stamp in the nonce permits an implementation which does not maintain state between transactions.¶

For applications where no possibility of replay attack can be tolerated the server can use one-time nonce values which will not be honored for a second use. This requires the overhead of the server remembering which nonce values have been used until the nonce time-stamp (and hence the digest built with it) has expired, but it effectively protects against replay attacks.¶

An implementation must give special attention to the possibility of replay attacks with POST and PUT requests. Unless the server employs one-time or otherwise limited-use nonces and/or insists on the use of the integrity protection of qop=auth-int, an attacker could replay valid credentials from a successful request with counterfeit form data or other message body. Even with the use of integrity protection most metadata in header fields is not protected. Proper nonce generation and checking provides some protection against replay of previously used valid credentials, but see 4.8.¶

An HTTP/1.1 server may return multiple challenges with a 401 (Authenticate) response, and each challenge may use a different auth-scheme. A user agent MUST choose to use the strongest auth-scheme it understands and request credentials from the user based upon that challenge.¶

• Note that many browsers will only recognize Basic and will require that it be the first auth-scheme presented. Servers should only include Basic if it is minimally acceptable.

When the server offers choices of authentication schemes using the WWW-Authenticate header, the strength of the resulting authentication is only as good as that of the of the weakest of the authentication schemes. See Section 4.8 below for discussion of particular attack scenarios that exploit multiple authentication schemes.¶

If the attacker can eavesdrop, then it can test any overheard nonce/response pairs against a list of common words. Such a list is usually much smaller than the total number of possible passwords. The cost of computing the response for each password on the list is paid once for each challenge.¶

The server can mitigate this attack by not allowing users to select passwords that are in a dictionary.¶

Both Basic and Digest authentication are vulnerable to "man in the middle" (MITM) attacks, for example, from a hostile or compromised proxy. Clearly, this would present all the problems of eavesdropping. But it also offers some additional opportunities to the attacker.¶

A possible man-in-the-middle attack would be to add a weak authentication scheme to the set of choices, hoping that the client will use one that exposes the user's credentials (e.g. password). For this reason, the client should always use the strongest scheme that it understands from the choices offered.¶

An even better MITM attack would be to remove all offered choices, replacing them with a challenge that requests only Basic authentication, then uses the cleartext credentials from the Basic authentication to authenticate to the origin server using the stronger scheme it requested. A particularly insidious way to mount such a MITM attack would be to offer a "free" proxy caching service to gullible users.¶

User agents should consider measures such as presenting a visual indication at the time of the credentials request of what authentication scheme is to be used, or remembering the strongest authentication scheme ever requested by a server and produce a warning message before using a weaker one. It might also be a good idea for the user agent to be configured to demand Digest authentication in general, or from specific sites.¶

Or, a hostile proxy might spoof the client into making a request the attacker wanted rather than one the client wanted. Of course, this is still much harder than a comparable attack against Basic Authentication.¶

With Digest authentication, a MITM or a malicious server can arbitrarily choose the nonce that the client will use to compute the response. This is called a "chosen plaintext" attack. The ability to choose the nonce is known to make cryptanalysis much easier [8].¶

However, no way to analyze the MD5 one-way function used by Digest using chosen plaintext is currently known.¶

The countermeasure against this attack is for clients to be configured to require the use of the optional "cnonce" directive; this allows the client to vary the input to the hash in a way not chosen by the attacker.¶

With Digest authentication, if the attacker can execute a chosen plaintext attack, the attacker can precompute the response for many common words to a nonce of its choice, and store a dictionary of (response, password) pairs. Such precomputation can often be done in parallel on many machines. It can then use the chosen plaintext attack to acquire a response corresponding to that challenge, and just look up the password in the dictionary. Even if most passwords are not in the dictionary, some might be. Since the attacker gets to pick the challenge, the cost of computing the response for each password on the list can be amortized over finding many passwords. A dictionary with 100 million password/response pairs would take about 3.2 gigabytes of disk storage.¶

The countermeasure against this attack is to for clients to be configured to require the use of the optional "cnonce" directive.¶

With Digest authentication, a MITM can execute a chosen plaintext attack, and can gather responses from many users to the same nonce. It can then find all the passwords within any subset of password space that would generate one of the nonce/response pairs in a single pass over that space. It also reduces the time to find the first password by a factor equal to the number of nonce/response pairs gathered. This search of the password space can often be done in parallel on many machines, and even a single machine can search large subsets of the password space very quickly -- reports exist of searching all passwords with six or fewer letters in a few hours.¶

The countermeasure against this attack is to for clients to be configured to require the use of the optional "cnonce" directive.¶

Basic Authentication is vulnerable to spoofing by counterfeit servers. If a user can be led to believe that she is connecting to a host containing information protected by a password she knows, when in fact she is connecting to a hostile server, then the hostile server can request a password, store it away for later use, and feign an error. This type of attack is more difficult with Digest Authentication -- but the client must know to demand that Digest authentication be used, perhaps using some of the techniques described above to counter "man-in-the-middle" attacks. Again, the user can be helped in detecting this attack by a visual indication of the authentication mechanism in use with appropriate guidance in interpreting the implications of each scheme.¶

Digest authentication requires that the authenticating agent (usually the server) store some data derived from the user's name and password in a "password file" associated with a given realm. Normally this might contain pairs consisting of username and H(A1), where H(A1) is the digested value of the username, realm, and password as described above.¶

The security implications of this are that if this password file is compromised, then an attacker gains immediate access to documents on the server using this realm. Unlike, say a standard UNIX password file, this information need not be decrypted in order to access documents in the server realm associated with this file. On the other hand, decryption, or more likely a brute force attack, would be necessary to obtain the user's password. This is the reason that the realm is part of the digested data stored in the password file. It means that if one Digest authentication password file is compromised, it does not automatically compromise others with the same username and password (though it does expose them to brute force attack).¶

There are two important security consequences of this. First the password file must be protected as if it contained unencrypted passwords, because for the purpose of accessing documents in its realm, it effectively does.¶

A second consequence of this is that the realm string should be unique among all realms which any single user is likely to use. In particular a realm string should include the name of the host doing the authentication. The inability of the client to authenticate the server is a weakness of Digest Authentication.¶

By modern cryptographic standards Digest Authentication is weak. But for a large range of purposes it is valuable as a replacement for Basic Authentication. It remedies some, but not all, weaknesses of Basic Authentication. Its strength may vary depending on the implementation. In particular the structure of the nonce (which is dependent on the server implementation) may affect the ease of mounting a replay attack. A range of server options is appropriate since, for example, some implementations may be willing to accept the server overhead of one-time nonces or digests to eliminate the possibility of replay. Others may satisfied with a nonce like the one recommended above restricted to a single IP address and a single ETag or with a limited lifetime.¶

The bottom line is that *any* compliant implementation will be relatively weak by cryptographic standards, but *any* compliant implementation will be far superior to Basic Authentication.¶